1. Field of the Invention
The present invention relates to a toner for forming a visible image from an electrostatic image formed on a surface of an image bearing member such as a photoconductor in electrophotography or electrostatic recording and also relates to an image forming apparatus using the toner.
2. Description of the Related Art
Electrophotography used in image forming apparatuses such as laser printers and dry electrostatic copiers includes uniformly charging a photoconductive insulation layer (charging step), exposing the layer to light, forming an electrical latent image by eliminating charges on the portions exposed to light (exposing step), forming a visible image by further attaching colored, charged fine powder, which is called a toner, to the latent image (developing step), transferring the visible image thus obtained onto a transfer material such as transfer paper (transfer step), and then permanently fixing the transferred image by an appropriate fixation method such as heating, pressurizing, or the like (fixing step).
In the fixing step, for example, commonly carried out is contact type heat fixing such as heating roller fixing in which a transferred image is fixed by heating and melting using a heating roller, and noncontact type heat fixing such as oven fixing. Recently, with an increased demand for energy conservation, a request is increased for reducing electric power consumption in a fixing step, which occupies a considerable part of the total electric power consumed by a copier. In this regard, the contact type heat fixing is preferred to the noncontact type heat fixing, because the contact type heat fixing is superior in heat efficiency, is capable of lowering a required fixing temperature as compared to a fixing temperature required for non-contact type heat fixing, and is effective in energy conservation and in downsizing of copiers.
However, in the contact type heat fixing method, an offset phenomenon is likely to occur disadvantageously, in which a part of toner melted at the time of fixing is transferred to a heating roller and then transferred to successive transfer paper sheets, etc. For prevention of this offset phenomenon, conventionally, the surface of a heating roller is coated with a material superior in releasing property such as fluorine resin, or a releasing agent such as silicone oil is applied to the surface of a heating roller. However, when silicone oil, etc. is used, the fixing device is required to be larger and more complex, leading to high production costs and causing frequent failures of the device.
Alternatively when a heating roller is used at a lowered temperature for prevention of the offset phenomenon, a toner is not sufficiently melted to cause an insufficient fixation because of the lowered temperature of the heating roller.
From the viewpoint of conserving energy and downsizing apparatuses, a toner is desired in which the offset occurrence temperature is high (hot-offset resistance) and the fixing temperature is low (low temperature-fixing property). Furthermore, the toner is further requested to have enough heat resistant storage stability so as not to undergo blocking during storage under the atmospheric temperature and humidity in the apparatus.
For solving these problems, for example, a toner binder is developed in which two types of resins with different softening points are mixed (see Japanese Patent Application Laid-Open (JP-A) No. 04-362956). However, in this method, as the mixing ratio of the amount of the resin with a lower softening point against the amount of the resin with the higher softening point is increased, blocking resistance and hot-offset resistance degrade, though low temperature-fixing property is improved, and in a region where the toner image is fixed at low temperatures, a difference between the softening temperature of the polyester with the lower softening temperature and the lower limit fixing temperature, becomes large, leading to an insufficient sharp melt property in the low temperature region. Therefore, in view of recent copiers' high-speed performance, down sizing, and saving of energy, a further improvement in low temperature-fixing property and offset resistance is desired.
Furthermore a toner is disclosed, which is obtained by dissolving and/or dispersing in an organic solvent a toner composition containing a modified polyester resin capable of reacting with an active hydrogen group-containing compound, and reacting the solution or the dispersion with a cross-linking agent and/or an elongating agent in an aqueous medium containing fine resin particles, in which a toner binder contains a crystalline polyester resin in addition to the modified polyester resin (see JP-A No. 2004-302458). In this case, since the crystalline polyester resin contained in the toner has crystallinity, the toner exhibits heat melting property that shows a sharp decrease in viscosity at a temperature close to the fixation beginning temperature, heat resistant storage stability is favorable because of the crystallinity immediately before the temperature at which the melting begins and at the melt beginning temperature, the viscosity of the toner sharply decreases (sharp melt property) and the toner is fixed. Therefore the proposal can provide a toner with excellent heat resistant storage ability as well as favorable low temperature-fixing property.
Further, an electrophotographic toner containing a toner binder and a colorant is disclosed, in which the toner binder contains three main components, that is, (A) a resin having a softening point in the range of 120° C. to 170° C., a glass transition temperature in the range of 58° C. to 75° C., and a rate of chloroform insoluble matter in the range of 5% by mass to 50% by mass, (B) a resin having a softening point in the range of 90° C. to 120° C. and a glass transition temperature in the range of 58° C. to 75° C., and (C) a crystalline polyester resin having a melting point in the range of 80° C. to 140° C.; the toner binder has a rate of chloroform insoluble matter of less than 30% by mass, and the toner contains a wax (W) having a penetration of 1.5 or less and a melting point in the range of 80° C. to 110° C. (see JP-A No. 2003-57875). In this case, to the two types of resins with different softening points, that is, resins (A) and (B), the resin (C) of crystalline polyester resin having a low melting point is melt-kneaded such that the resin (C) is uniformly dispersed in the resins (A) and (B) to thereby produce a toner excellent in hot-offset resistance and blocking resistance while maintaining excellent low-temperature fixing property.
A toner resin composition containing a crystalline polymer having a melting point in the range of 180° C. to 280° C. in which an absorption amount of heat at the melting point measured by a differential scanning calorimeter (DSC) is in the range of 25 mJ/mg to 150 mJ/mg, and containing a noncrystalline polyester resin having a glass transition temperature in the range of 30° C. to 80° C. is disclosed, wherein the noncrystalline polyester resin contains first noncrystalline polyester resin having weight average molecular weights in the range of 3,000 to 20,000 and second noncrystalline polyester resin having weight average molecular weights in the range of 30,000 to 300,000 (see JP-A No. 2004-151709). When this toner resin composition is used, a toner can be prepared which is excellent in low temperature-fixing property, hot-offset resistance, and blocking resistance and enables favorable coloring. In this case, physically cross-linked structure is formed among crystalline components of the crystalline polymer having a high melting point in the noncrystalline polyester resin, meantime, the noncrystalline components in the crystalline polymer having a high melting point are entangled with the noncrystalline polyester resin to form a sort of network structure. By forming such a network structure, the toner makes it possible to exhibit favorable offset resistance without causing a substantial reduction in viscosity at high temperatures and without decreasing low temperature-fixing property and storage stability.
Furthermore, a toner is disclosed, in which, when a toner binder is composed of at least a toner binder component LR and a toner binder component HR, Tlr3 (° C.) is a temperature at which the melt viscosity characteristics of the toner binder component LR are 1×103 (Pa·S), Thr3 (° C.) is a temperature at which the melt viscosity characteristics of the toner binder component HR are 1×103 (Pa·S), and Tcr3 (° C.) is a temperature (a peak temperature of heat absorption measured by a DSC method) at which the melt viscosity characteristics of a crystalline resin component CR are 1×103 (Pa·S) (see JP-A No. 2005-164800), Tlr3 is in the range of 90° C. to 125° C., Thr3 is in the range of 155° C. to 210° C., and Tcr3 is in the range of 95° C. to 150° C. In this case, when the toner binder contains resin components having the above characteristics, a toner can be obtained which provides a favorable image quality, a wide range of fixing temperatures, and also favorable blocking resistance.
In the above related art, even when a plurality of resins having significantly different softening point ranges are mixed, by adding a crystalline polyester resin into one or more types of noncrystalline resins, each of original resin characteristics are maintained in each toner. However, in these cases, after crystalline polyester resin is added into noncrystalline polyester resin and the mixture is once heated, the noncrystalline polyester resin and the crystalline polyester resin dissolves each other, which results in inability to obtain each of the original resin characters. As a result, toner with satisfactory blocking resistance and image quality is difficult to obtain. Furthermore, in the related art, an organic or inorganic tin compound, which is environmentally problematic, is sometimes used as a catalyst for synthesis of polyester of a raw material for toner.